The present invention relates to a ballast circuit for operating a discharge lamp, and more particularly to an electronic discharge lamp ballast having a circuit protection against an end of lamp-life condition.
In order to protect an electronic ballast from an excessive current flowing through the ballast circuit as a result of the lamp reaching its end of life condition, the prior art electronic ballasts are designed to detect the end of lamp life and restrict the ballast operation once the end of life is detected. A typical example of the electronic ballast having such protection is disclosed in Japanese patent publication No JP11-31594. The ballast of this publication utilizes a voltage detector which detects a lamp voltage, i.e., a voltage across the lamp for determination of whether the lamp voltage exceeds a predetermined threshold as indicative of that the lamp reaches the end of life. However, the detector of the ballast is required to withstand a high voltage applied at the start of the lamp and therefore has to adopt resistors and like elements capable of withstanding the high-voltage, resulting in increased cost of the ballast and a somewhat bulky assembly. The problem is solved in another prior art, i.e., U.S. Pat. No. 5,925,990 in which the detector is arranged to detect a voltage across a capacitor which is inserted in the ballast in series with the discharge lamp. Although this prior art is successful in avoiding the high voltage being applied to the detector, the detector has to be configured to derive a variation width of the voltage appearing across the capacitor in order to determine the end of lamp life. This scheme of detecting the lamp life end requires a rather complicated circuit arrangement with an attendant increase in manufacturing cost. Further, with the inclusion of the capacitor in series with the discharge lamp, both output terminals for the discharge lamp are always made to have a high potential relative to a ground line of the ballast circuit, which requires an additional hazard protection of avoiding electric shocks when replacing the discharge lamp.
In view of the above insufficiencies, the present invention has been accomplished to provide a ballast circuit for the discharge lamp which is capable of protecting the circuit when the lamp reaches its end of life with a simple and cost effective circuit arrangement, yet assuring a safe lamp replacement. The ballast circuit in accordance with the present invention comprises a DC voltage supply providing a driving DC voltage, a pair of first and second inverter switches, and a series resonant circuit generating and applying a high frequency resonant voltage to the discharge lamp. The first and second inverter switches are connected in series across the DC voltage supply and are driven to turn on and off alternately. The first inverter switches defines a high-side switch and the second inverter switch defines a low-side switch having one end connected to a ground line of the ballast circuit. The resonant circuit is composed of an inductor and a capacitor, and is connected across the second inverter switch through a blocking capacitor so as to generate the high frequency resonant voltage in response to the alternate turn on and off of the first and second inverter switches. The capacitor of the resonant circuit is adapted to be connected across the discharge lamp for applying the resonant voltage thereto. A voltage comparator is provided to detect a DC voltage appearing in the ballast circuit as a consequence of the discharge lamp reaching its lamp-life-end, and compares the detected DC voltage with a predetermined threshold and generates a lamp-life-end signal when the DC voltage exceeds the threshold. Connected to the voltage comparator is a controller which receives the lamp-life-end signal and controls the first and second inverter switches in order to reduce or stop an output power being fed to the discharge lamp. The feature of the present invention resides in that a DC sensing capacitor is connected in series with a resistor and the blocking capacitor across the second inverter switch in order to detect the DC voltage, and that the DC sensing capacitor is connected in series with the resistor and the inductor of the resonant circuit across the capacitor of the resonant circuit with one end of the DC sensing capacitor being connected to the ground line of the ballast circuit. Since the DC sensing capacitor is connected in series with the blocking capacitor outside of the resonant circuit, it can be kept free from the high voltage being generated by the resonant circuit and applied to the lamp at the start of the lamp. In this sense, the DC sensing capacitor and its associated parts have not to withstand the high voltage and therefore can be of less cost for reducing the manufacturing cost of the ballast. Further, since the DC sensing capacitor is connected in parallel with the capacitor of the resonant circuit with the one end of the DC sensing capacitor being connected to the ground line of the ballast circuit, the DC voltage detected at the DC sensing capacitor itself can directly indicate whether or not the lamp reaches the end of life, i.e., simply by comparing the DC voltage itself with the threshold and not requiring to evaluate a variation width of the detected voltage, thereby simplifying the circuit arrangement for determining the lamp life end also for reducing the manufacturing cost, in addition that the one end of the discharge lamp can be held at the ground potential for reducing safety hazard at the time of replacing the lamp.
Accordingly, it is a primary object of the present invention to provide an improved ballast circuit for operating the discharge lamp which is capable of protecting the circuit against the lamp life end condition, while reducing the manufacturing cost and assuring safe replacement of the lamp.
The ballast circuit may be designed to operate at least two discharge lamps. For this purpose, the ballast circuit includes at least two series resonant circuit each composed of an inductor and a capacitor, and connected in series with a blocking capacitor across the second inverter switch. The at least two resonant circuits are connected in parallel with each other across the second inverter switch for providing the resulting resonant voltage. The capacitor of each resonant circuit is adapted to be connected across each of the discharge lamp. Also included in the ballast circuit are at least two DC sensing capacitors each connected in series with a resistor and each of the blocking capacitors across the second inverter switch. The voltage comparator is connected to the at least two DC sensing capacitors and provide the lamp-life-end signal when the DC voltage detected at any one of the DC sensing capacitors exceeds the threshold.
In another version where at least two resonant circuits are provided to operate at least two discharge lamps, the resonant circuits are connected in series commonly with the blocking capacitor across the second inverter switch. A single DC sensing capacitor is connected in series with a resistor and the common blocking capacitor across the second inverter switch, and provided the DC voltage indicative of whether or not any one of the lamps reaches the end of life. Thus, only one DC sensing capacitor is sufficient for determination of the lamp-life-end for any one of the discharge lamps, simplifying the circuit design.
Preferably, a bypass resistor is connected in series with the blocking capacitor across the first inverter switch in order to detect the lamp-life-end due to a slow-leak of a gas from the discharge lamp. When the slow-leak occurs so that the lamp cannot sustain the operation, the bypass resistor allows the blocking capacitor to release a current through the second inverter switch, the DC voltage supply and the bypass resistor, thereby accumulating the DC voltage across the DC sensing capacitor. Thus, the lamp-life-end can be successfully detected by the presence of the DC voltage across the DC sensing capacitor.
The ballast circuit of the present invention is preferred to include an inverter controller which drives the first and second inverter switches to turn on and off at a varying frequency. In this connection, the ballast circuit may include a dimmer controller which generates a dimming signal in response to an external dimmer command, and a frequency controller which varies the frequency of the inverter controller in response to the dimmer signal for adding a dimming control of the lamp.
These and still other advantageous features of the present invention will become more apparent from the following description of the preferred embodiments when taken in conjunction with the attached drawings.